Holographic storage system incorporated therein a parabolic mirror

ABSTRACT

A holographic storage system implements both a rotational and an angular multiplexings in combination. The system comprises a light source, a pair of wedge prisms, a beam splitter, a first, a second, a third and a parabolic mirrors, a SLM(spatial light modulator) and a storage medium. In the system, a light beam emitted from the light source is split into a reference and a signal beams by the beam splitter. The signal beam reflected by the first mirror to the SLM which modulates data in the firm of pages. The modulated signal beam falls into the storage medium, while the reference beam is reflected by the second mirror to the wedge prisms which deflect the reference beam to an arbitrary position on the third mirror. The deflected reference beam is reflected onto the parabolic mirror by the third mirror. The deflected reference beam is converged onto the storage medium by the parabolic mirror. In the storage medium, the modulated signal beam interferes with the converged reference beam to thereby generate an interference pattern therebetween, wherein the interference pattern contains information of the modulated signal beam. The holographic storage system has a size reduced by incorporating therein the third mirror and the parabolic mirror provided with an opening.

FIELD OF THE INVENTION

The present invention relates to a holographic storage system; and, moreparticularly, to a holographic storage system with a size reduced byincorporating therein a parabolic mirror.

DESCRIPTION OF THE PRIOR ART

As is well known, demands for optically storing a large amount of datain such cases as a motion picture film have been increasing. Therefore,various types of holographic storage systems incorporating therein astorage medium have been developed for realizing high density opticalstorage capabilities, wherein the storage medium is conventionally madeof lithium niobate(LiNbO₃) or lithium borate(Li₂ B₂ O₄) and is used forthree-dimensionally storing the data in the form of pages.

An angular-multiplexed storage system is most commonly used among theseholographic storage systems since a variation in angles can be easilyobtained by rotating the storage medium or by deflecting a light beam tobe used for writing or reading the data stored in the storage medium.The angular-multiplexed storage system comprises a laser source forgenerating a coherent light beam, a beam splitter, a first and a secondmirrors, a detector and a storage medium in the form of a rectangularhexahedron or rectangular plate. In the system, the coherent light beamfrom the laser source enters into the beam splitter which splits thecoherent light beam into a reference and a signal beams. The referenceand the signal beams are reflected to the storage medium by the firstand the second mirrors, respectively. Interference patterns generated bythe signal and the reference beams are recorded into the storage medium.Therefore, the angular-multiplexed storage system is capable of writingand reading the data into/from the storage medium.

One way for this angular-multiplexed storage system to address the datais achieved by way of controlling the direction of the reference beamonto a specific region within the storage medium. This is typically donethrough the mechanical movement of mirrors or lenses. However, thismethod requires that the positions of the mirrors or the lenses forreading be precisely aligned with those for writing.

Another way, known as a rotational multiplexing, is implemented bydirectly rotating the storage medium about an axis perpendicular to thesurface thereof, wherein the axis lies on a plane which includes thereference and the signal beams. Recently, the idea of combining thesemultiplexing techniques is gaining popularity so as to record a muchlarger amount of holograms in the storage medium. However, therotational multiplexing is not adaptable for combination with othermultiplexing methods such as spatial multiplexing which is implementedby shifting the storage medium, since different mechanical motionsshould be implemented to control both the reference beam and the storagemedium. And, the implementation of different mechanical motions is oftendifficult, because, for instance, the rotational stage on which thestorage medium is placed should be mounted, in turn, on a translationalstage.

Further, one of the common shortcomings of the above-describedmultiplexing techniques is a large size thereof due to the use of anobjective lens which is located in front of the storage medium forconverging a reference beam on the storage medium, thereby making theoverall size of the holographic storage system bulky.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to providean improved holographic storage system capable of storing large amountsof hologram data into a storage medium by rotating a wedge prismincorporated therein.

In accordance with the present invention, there is provided aholographic storage system for storing/reading multiple holograms, thestorage system comprising: a light source; a beam splitter for splittinga light beam emitted from the light source into a reference and a signalbeams; a storage medium for storing the multiple holograms thereinto; afirst mirror for reflecting the signal beam to the storage medium; asecond and a third mirrors; a pair of wedge prisms for deflecting thereference beam, which is reflected to the wedge prisms by the secondmirror, to the third mirror and rotating the deflected reference beam byrotating the wedge prisms; and a parabolic mirror for converging thedeflected reference beam from the third mirror onto the storage medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention together with the above and other objects andadvantages will become apparent from the following description ofpreferred embodiments, when given in conjunction with the accompanyingdrawings, wherein:

FIG. 1 shows a schematic diagram of a holographic storage system inaccordance with the present invention; and

FIGS. 2A to 2D illustrate explanatory diagrams of a light path in a pairof wedge prisms incorporated into the holographic storage system shownin FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, there are illustrated a holographic storagesystem in accordance with a preferred embodiment of the presentinvention. It should be noted that like parts appearing in FIGS. 1 and 2are represented by like reference numerals.

There is illustrated in FIG. 1 a schematic diagram of a holographicstorage system 100 in accordance with a preferred embodiment of thepresent invention. The holographic storage system 100 comprises alightsource 102 for generating a coherent light beam, a beam splitter 104, afirst, a second and a third mirrors 106, 108, 136, a parabolic mirror130 provided with an opening 132, a first and a second controllers 140,142, a storage medium 150, a Fourier transform lens 160, a SLM(spatiallight modulator) 170, a detection lens 180, an optical detector 190 anda first and a second wedge prisms 110, 120, wherein each of the wedgeprisms 110, 120 having a slope and a normal surface. The opticaldetector 190 includes an array of M×N elements, each of the elementsbeing capable of detecting a light beam incident thereto.

In the system 100, the coherent light beam emitted from the light source102, e.g., a semiconductor laser, travels to the beam splitter 104. Thebeam splitter 104 partially reflects the coherent light beam to therebysplit the coherent light beam into a reference beam and a signal beam,wherein the signal beam is a portion of the coherent light beamtransmitted to the first mirror 106 through the beam splitter 104 andthe reference beam is a remaining portion of the coherent light beamreflected to the second mirror 108 by the beam splitter 104. The storagemedium 150 is made of a photorefractive crystal such as lithiumniobate(LiNbO₃).

And then, the signal beam falls onto the first mirror 106 which reflectsthe signal beam to the SLM 170. In the first preferred embodiment of thepresent invention, the SLM 170 includes discrete modulating regions,e.g., an array of M×N modulating pixels, M and N being positiveintegers, respectively. Each of the M×N modulating pixels is controlledby a voltage applied thereto through an integrated circuit(not shown),whereby the SLM 170 controls an amplitude and a phase of the signal beamimpinged onto each of the M×N modulating pixels. Therefore, the SLM 170is capable of converting the signal beam impinged thereonto into amodulated signal beam which contains data in the form of page after thesignal beam passing therethrough. The modulated signal beam is impingedonto the Fourier transform lens 160 which converges the modulated signalbeam on a recording area of the storage medium 150. In case that theFourier transform lens 160 is not used, the modulated signal beam canalso be recorded on the recording area of the storage medium 150,directly.

In the meantime, the reference beam enters into the second mirror 108which reflects the reference beam to the first wedge prism 110. Afterthe reference beam enters the first wedge prism 110, the optical paththereof in the first and the second wedge prisms 110, 120 will bedescribed in detailed hereinafter. In the preferred embodiment, thereference beam is arranged in such a way that it is inclined at an angleθ_(i) of incidence with respect to Z-axis. It is preferable that θ_(i)is approximately 0 degree. And also, in case when the θ_(i) is slightlylarger or less than a slanted angle θ_(w1), the holographic storagesystem 100 also can record pages of data into the storage medium 150,wherein θ_(w1) represents an angle between the normal surface 112 andthe slope 114 of the first wedge prism 110 as shown in FIG. 2A.

Referring to FIG. 2A, there is shown an optical path of the referencebeam in the first wedge prism 110 shown in FIG. 1. For the sake ofexplanation, suppose the normal surface 112 of the first wedge prism 110is perpendicular to the Z-axis. When the reference beam enters thenormal surface 112 with an angle θ_(i) of incidence, the reference beamis transmitted into the first wedge prism 110 with a Transmitted angleθ₁ '. The angle θ₁ ' can be obtained by using the followingrelationship:

    n.sub.w1 sinθ'.sub.1 =nsinθ.sub.i              Eq.(1)

wherein n_(w1) represents a refractive index of the first wedge prism110, and n, the refractive index of the air. If the θ_(i) is much lessthan π/2, θ₁ '≈θ_(i) /n_(w1). A point on the slope 114 where thetransmitted reference beam arrives at is denoted by O. The two straightlines that pass through the point O are denoted by I-I' and II-II',where I-I' is in parallel with Z-axis and II-II' is normal to the slope114. If an angle IOII(=II'OI') is represented by θ₁, then θ₁ =θ_(w1).The transmitted reference beam impinges onto the slope 114 with anangle(θ₁ +θ₁ ') of incidence at a point O and exits the wedge prism 110with an angle θ_(d) at the slope 114 of the wedge prism 110. Therefore,if an angle between the direction of the exited reference beam andZ-axis(or I-I') is given by θ₂, then θ_(d) =θ₁ +θ₂. From Snell's law, weobtain n_(w1) sin (θ₁ +θ₁ ')=sin (θ₁ +θ₂) or n_(w1) sin (θ_(w1) +θ_(i)/n_(w1)) =sin (θ_(w1) +θ₂). For θ_(w1) <<π/2, approximately n_(w1)θ_(w1) +θ_(i) ≈θ_(w1) +θ_(d). The angle θ₂ can be written as followingrelationship:

    θ.sub.2 ≈(n.sub.w1 -1)θ.sub.w1 +θ.sub.iEq.(2)

Suppose the first wedge prism 110 rotates about Z-axis. If the secondwedge prism 120 is located in the (x, y) plane separated from the normalsurface 112 of the first wedge prism 110 at a distance L as shown inFIG. 2B, the trajectory P(x, y) of the exited reference beam will form acircle on the normal surface 122 of the second wedge prism 120. The xand y values of P are given by following relationship:

    x+jy=rexp (jδ.sub.1)                                 Eq.(3)

wherein j is an imaginary unit, r=Ltanθ₂ ≈Lθ₂, and δ₁ is a rotationangle of the first wedge prism 110 from X-axis.

Referring to FIG. 2C, there is shown an optical path of the exitedreference beam in the second wedge prism 120 shown in FIG. 1. A totaldeflection angle θ_(dt) from Eq. (2), after passing through the firstand the second wedge prisms 110, 120, is given by followingrelationship:

    θ.sub.dt ≈(n.sub.w2 -1)θ.sub.w2 +(n.sub.w1 -1)θ.sub.w1                                         Eq.(4)

because an angle of incidence at the normal surface 122 of the secondwedge prism 120 is equal to θ₂ of the first wedge prism 110 and θ_(i) isapproximately equal to 0. Suppose each of the wedge prisms 110, 120 isrotated about Z-axis by δ₁ and δ₂, respectively. Then, the x and yvalues of the output beam position P at the third mirror 136 at adistance D from a center point between the two wedge prisms 110, 120 canbe written as following relationship:

    x+jy=r.sub.1 exp (jδ.sub.1)+r.sub.2 exp(jδ.sub.2)Eq.(5)

For example, if D is much larger than L and the two wedge prisms 110,120 are identical to each other in shape, both of r₁ and r₂ areapproximately equal to D₁ θ₂. Therefore, the exited reference beam,after passing through the opening 132 of the parabolic mirror 130 andthe second wedge prism 120, can be deflected to an arbitrary positionwithin a circle with a radius 2r₁ on the third mirror 136 by controllingδ₁ and δ₂ properly, thereby obtaining a deflected reference beam. InFIG. 2D, if the two wedge prisms 110, 120 rotate with angular velocitiesw₁ and w₂, respectively, the trajectory P(x, y) of the deflectedreference beam on the third mirror 136, after passing through theopening 132 of the parabolic mirror 130, is given by

    x(t)+jy(t)=r.sub.1 e.sup.[j(w.sbsp.1.sup.t+δ.sbsp.1.sup.)] +r.sub.1 e.sup.[j(w.sbsp.2.sup.t+δ.sbsp.2)]                  Eq.(6)

For example, if δ₁ ≠δ₂ and w₁ =w₂, the deflected reference beam rotatescircularly with a radius of 2r₁ Cos[(δ₂ -δ₁) /2] and the angularvelocity w₁. In addition, if δ₁ =δ₂ and w₁ =-w₂, a straight-line motionis obtained in the range of -2r₁ ≦x≧2r₁, because x(t)=2r₁ cos(wt),y(t)=0 in Eq. (6). In general, any kind of motions, such as elliptical,spiral, and concentric circular ones, can be obtained by properlycontrolling δ₁, δ₂, w₁ and w₂.

Referring back to FIG. 1, the deflected reference beam is reflected tothe parabolic mirror 130 by the third mirror 136. The parabolic mirror130 converges the deflected reference beam impinging thereto on arecording area of the storage medium 150. In the recording area, themodulated signal beam interferes with the converged reference beam tothereby generate an interference pattern therebetween, wherein theinterference pattern contains information of the modulated signal beam.

Although the parabolic mirror 130 in the preferred embodiment is aparabola having an opening 132, the shape of the parabolic mirror 130 isnot limited to the parabola. As long as a light beam, after passingthrough the opening 132, is converged on the storage medium 150, anothermirror such as a semi-circle mirror provided with an opening and agrating and the like can also be used for realizing the multiplexing.

If another page of data modulated by the SLM 170 is to be recorded onthe recording area of the storage medium 150, the first and the secondcontrollers 140, 142 rotate the wedge prisms 110, 120 with the angularvelocities w₁ and w₂, respectively, so that the holographic storagesystem 100 realizes both a rotational and an angular multiplexings incombination.

Thereafter, when reading the stored data, the modulated signal beamretrieved from the storage medium 150 enters the optical detector 190which is capable of detecting the power of the retrieved signal beam.The retrieved signal beam is generated by diffracting the reference beamfrom the storage medium 150.

While the present invention has been described with respect to thepreferred embodiments, other modifications and variations may be madewithout departing from the spirit and scope of the present invention asset forth in the following claims.

What is claimed is:
 1. A holographic storage system for storing/readingmultiple holograms, the storage system comprising:a light source; a beamsplitter for splitting a light beam emitted from the light source into areference and a signal beams; a storage medium for storing the multipleholograms therinto; a first mirror for directing the signal beam towardsthe storage medium; a second mirror for directing the reference beamtowards the storage medium; a rotatable wedge prism for deflecting thedirected reference beam in a circular pattern; and a parabolic mirror,provided with an opening and a third mirror, for converging thedeflected reference beam on the storage medium.
 2. The storage system ofclaim 1, wherein the parabolic mirror is disposed between the rotatablewedge prism and the storage medium.
 3. The storage system of claim 2,wherein the third mirror is disposed between the parabolic mirror andthe storage medium.
 4. The storage system of claim 3, wherein thereference beam deflected from the rotatable wedge prism is reflected tothe parabolic mirror by the third mirror after passing through theopening of the parabolic mirror.
 5. The storage system of claim 4,wherein the third mirror is in the form of a disk.
 6. The storage systemof claim 1, wherein the rotatable wedge prism includes a first and asecond prisms.
 7. The storage system of claim 6, wherein each of theprisms is in the form of a wedge.
 8. The storage system of claim 7,wherein each of the prisms includes a flat surface and a slope which isinclined at a predetermined angle with respect to the flat surface. 9.The storage system of claim 8, wherein the first prism is disposedbetween the parabolic mirror and the light source.
 10. The storagesystem of claim 9, wherein the second prism is disposed between theparabolic mirror and the first prism.
 11. The storage system of claim10, wherein an incidence angle of the reference beam onto the normalsurface of the first prism ranges from zero to an angle between thenormal surface and the slope of the first prism.
 12. The storage systemof claim 11, further comprising a first controller for rotating thefirst prism with a first angular velocity w₁ and a second controller forrotating the second prism with a second angular velocity w₂, whereby thereference beam incident to the first prism is deflected to the arbitraryposition on the mirror by rotating the wedge prisms.
 13. The storagesystem of claim 12, further comprising means for modulating the signalbeam impinged thereonto into a modulated signal beam which contains datain the form of pages after passing therethrough, the modulating meansbeing placed between the storage medium and the directing means.
 14. Thestorage system of claim 13, wherein, if another page of the hologram ismodulated by the modulating means, the controllers change a position ofthe reference beam on the parabolic mirror by a predetermined amount.15. The storage system of claim 14, wherein, if w₁ is equal to w₂, aposition of the reference beam impinging onto the parabolic mirror movesalong a circle with a constant radius.
 16. The storage system of claim14, wherein, if w₁ is equal to -w₂, a position of the reference beamimpinging onto the parabolic mirror moves along a straight line.
 17. Thestorage system of claim 14, further comprising means for moving thestorage medium for a spatial multiplexing.